Welding techniques include those performed by friction, such as so-called “friction stir welding” (FSW). Such a technique is advantageous for assembling together metal parts that are thin, e.g. presenting thickness of less than about 1.5 millimeters (mm). By way of example, such parts are constituted by plates, sheets, or other analogous parts that are thin and of large dimensions in the general plane in which they extend. The parts may be plane parts or they may be shaped, and their edges that are used for welding them together are considered as being generally plane in the welding zone. In general terms, the friction-welding technique consists in holding together the parts that are to be assembled by means of specific tooling. More particularly, the parts are held adjacent one another and pressing one against the other, e.g. in abutment so as to put their corresponding edge faces into contact one against the other, or with one of the edges of a first part overlapping the edge of a second part. The first part is thicker than the second part and it includes a rabbet or analogous feature for receiving the corresponding edge of the second part.
With the parts held in position for welding by the tooling, a chuck carrying a welding pin is used to cause the adjacent edges of the part to be heated. The welding pin is provided with roughness in relief formed at its free end remote from its end that is engaged in the chuck. By way of example, this roughness in relief is a thread or is the result of the free end of the welding pin being given a section of polygonal shape. Putting the welding pin into contact with the adjacent edges of the parts and then moving it relative thereto serves to cause a bead of welding to be formed, thereby assembling the parts together. The bead of welding is obtained in the welding zone as a result of the parts heating under the effect of the friction applied locally thereto by the welding pin. This heating causes the materials constituting the parts to take on a pasty state, which materials then mix together by spreading. The parts are subsequently joined together by continuous dynamic recrystallization of the material in the welding zone.
In a first technique for holding the parts, their edges are held pressed adjacently against an anvil by clamping or by some other analogous technique. Clamping members press at least one of the parts against opposing thrust received by one and/or the other of the parts against an anvil. The term “anvil” is used to designate any member suitable for forming a bearing surface or analogous bearing member against which the parts are pressed by clamping, whether directly or indirectly.
In a second technique for holding the parts, the edges of the parts are held in their relative position by means of a pair of spools bearing respectively against one and the other of the opposite faces of the parts to be assembled together. More particularly, a bottom spool is fitted with the welding pin arranged as a threaded rod that is secured to a top spool. Clamping the edges of the parts between the spools enables them to be docked and held together in localized manner in the welding zone.
Furthermore, in the field of aviation, it is desirable to be able to make a hollow structure that is made up of a plurality of parts that are assembled together one after another. Such a hollow structure constitutes in particular an airfoil, a tail unit, or some other analogous hollow structure. For example, two component parts of the hollow structure may be preformed by being folded in half so as to enable them to be shaped, and they are abutted via a third part that is arranged as a transversal section for transversally stiffening the hollow structure. Each preformed part presents a generally plane profile of the hollow structure, one of the parts constituting a leading profile and the other a trailing profile. The section has an I-shaped section or the like with end flanges depending on the profile of the section. The section is interposed between the facing edges of the leading profile and of the trailing profile, said section being oriented orthogonally relative to the general plane in which the preformed parts extend. The flanges of the section include rabbets for receiving respective corresponding edges of the parts, so as to make it easier to position them relative to one another.
Traditionally, a riveting technique is used for assembling the preformed parts with the section.
Nevertheless, it is desirable for the rivets to be flush with the outside face of the hollow structure, and this is difficult to achieve when the desired preformed parts are of small thickness, e.g. of thickness less than about 1.5 mm. Although the friction welding assembly technique is advantageous for use in assembling preformed parts with the section, it is difficult to implement in this context. Applying the welding pin against the edges of the preformed parts gives rise to large tensions and vertical forces in the general plane in which the parts extend. The quality and the reliability of the resulting assembly are uncertain, and holding the preformed parts in shape during the welding operation is difficult to achieve, particularly since the hollow structure is made up of preformed parts that are thin and that present a cross section of height that is small relative to length in the general plane in which they extend.
The first above-mentioned holding technique is more particularly suitable for use in assembling plane or curved parts having adjacent edges that are held in abutment via their corresponding edge faces, or else in overlap. The parts are held firmly and effectively over their entire length throughout the complete welding operation serving to form the bead of welding. However, at present, that holder tooling is massive and although it is effective for parts that are plane or curved, it is not suitable in its current state for assembling together parts that enable a hollow structure of the abovementioned kind to be made.
The second above-mentioned technique for holding the parts runs the risk of the parts overheating because they are engaged between the spools and because of the resulting friction. When assembling together parts of small thickness, thickness of the order of less than 1.5 mm, such heating tends to give rise to local spoiling of the properties of the material constituting the parts, and can even make it impossible to obtain continuous dynamic recrystallization of the resulting bead of welding. In addition, localized holding of the preformed parts at their corresponding edges does not guarantee that they are kept in shape, and therefore does not guarantee that a hollow structure will be obtained that is made up of the desired profiles.
Furthermore, document US 2004/050907 describes using a plurality of anvils and pressure backing-tools in order to hold the parts for assembly in position in the vicinity of the connection that is to be made.
Document JP 10230375 describes using anvil systems so as to avoid the parts moving apart during friction welding.
Document JP 11058038 describes a device that provides more thorough holding of the structure. In contrast, document US 2005/045693 provides for localized holding of the parts.
Finally, document JP 61146430 provides for the use of two frames.